The prior lead-acid battery charger of the electric bike generally has a three-stage charging mode, i.e. constant current charging, constant voltage charging, and constant voltage trickle charging. Always a direct current is output and the battery will be polarized during charging. Moreover, the voltage value and the current value during charging are given under a temperature condition of 25° C. As the charging for the lead-acid battery is remarkably affected by the air temperature, the battery would be over-charged in summer and would be under-charged in winter. Both the over-charging and under-charging would cause water loss and sulfation of the lead-acid battery.